1. Field of the Invention
This invention pertains to polyurethanes and compositions including same useful as surface coatings, the polyurethanes having a plurality of pendant sterically hindered amines and a plurality of pendant moieties which allow the polyurethanes to be emulsified.
2. Related Art
Polyurethane surface coatings are used when surfaces require abrasion resistance and flexibility. In addition, it is preferable that the polyurethane be fast curing, exhibit good adhesion to the substrate to which it is coated, and yield a chemically resistant coating.
The addition polymerization of diisocyanates with macroglycols to produce polyurethanes from liquid monomers is well known. Polyurethanes contain carbamate groups (--NHCOO--), also referred to as urethane groups, in their backbone structure. These are obtained by the reaction of an isocyanate with a macroglycol, also a so-called polyol, or with a combination of a macroglycol and a short-chain extender. In the latter case, segmented block copolymers are produced. Polyols used in these coatings are typically either polyester polyols or polyether polyols. The isocyanates, and isocyanate prepolymers made therefrom, have frequently utilized 2,4-toluene diisocyanate because of the difference in reactivity of the two isocyanate groups. Aliphatic isocyanates, for example methylene bis(cyclohexyl isocyanate) (H.sub.12 MDI), the biuret of hexamethylene diisocyanate, and isophorone diisocyanate (IPDI), offer color stability not provided by aromatic isocyanates.
Moisture-cured polyurethane coatings are isocyanate-terminated prepolymers which, after application, are cured by reaction of the residual isocyanate groups with moisture. The amino groups initially formed react with more isocyanate groups to form urea linkages. Such coatings are typically applied as architectural finishes.
For external applications, aliphatic isocyanates are preferred if the coating is to remain colorless, since coatings based on aromatic isocyanates discolor when exposed to ultraviolet radiation for long time periods. In particular, H.sub.12 MDI is known to be used in the formulation of soft elastomeric coatings and clear finishes for resilient vinyl flooring and elastomeric automotive coatings. See generally Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Ed., John Wiley & Sons, vol. 23, pages 576-608.
There are many patents which describe silane-terminated polyurethanes. The majority of these compositions are based on moisture curable formulations for sealants. These include U.S. Pat. Nos. 3,632,557; 3,903,052; 3,979,344; 4,020,043; 4,222,925; and 4,345,053. Most if not all of the polyurethanes described are moisture-curable formulations for sealants.
Water-borne silane-terminated polyurethanes are known, such as those described in U.S. Pat. No. 3,983,291 (Chang). U.S. Pat. No. 4,567,228 (Gaa et al.) describes aqueous dispersible, isocyanate-containing polymers, prepolymers, and chain extended polymers having internal pendant, reactable silane groups. By having internal (i.e. in the polymer backbone) silane groups, Gaa et al. apparently overcome problems exhibited by coating mixtures having polyurethane resins and organosilane coupling agents in mechanical mixture which, upon application to a substrate, react independently with the substrate. One problem mentioned is that the final deposition of the organosilane and polyurethane in the coating on the substrate is somewhat random, leading to some of the organosilane associating with the surface before associating with the polyurethane, apparently leading to poor adhesion of the coating to the surface. Polyurethane resins having internal pendant silylation apparently avoid this problem.
Unfortunately, the above polymers are not readily removed from substrates with aqueous compositions. Such polymers have found utility as "permanent" coatings on concrete, for example. Typically they have not enjoyed utility on vinyl floors in part because the techniques and chemicals used to remove them would also tends to damage a vinyl floor.
Acrylic coatings are known which comprise acrylic polymers having pendant COO.sup.- moieties which are neutralized by tertiary amines. When combined with a Zn.sup.2++ containing compound, two tertiary amines are displaced by one Zn.sup.2++ ion to form a plurality of coordination sites which effectively crosslinks the composition. This crosslinking can be reversed by exposing the coating to an aqueous amine, but not by acidic compositions. However, typical hard surface cleaning chemicals are alkaline in order to effectively remove grease substances from the surface. Alkaline cleaning chemicals may actually dull the surface of a zinc-crosslinked polymer because they tend to remove a small amount of coating from the surface. If it is desired to maintain a glossy surface, the cleaned-and-dulled coating would have to be polished to restore the luster.
U.S. Pat. No. 5,073,195 describes silane coupling agents having the general formula A.sub.(4-n) SiY, wherein A is a monovalent organic radical, Y is a hydrolyzable radical, and n is 1, 2, or 3.
Despite the presence of the above compositions, to the inventor's knowledge, previously known aqueous silane-terminated polyurethanes are not easily removed with any known aqueous composition. Therefore, there continues to be a need in the art of polyurethane coatings for an aqueous silane-terminated polyurethane composition that can be removed easily with aqueous compositions.